Gas sensor device for detecting gases with large molecules

ABSTRACT

The present disclosure is directed to a gas sensor device that detects gases with large molecules (e.g., a gas with a molecular weight between 150 g/mol and 450 g/mol), such as siloxanes. The gas sensor device includes a thin film gas sensor and a bulk film gas sensor. The thin film gas sensor and the bulk film gas sensor each include a semiconductor metal oxide (SMO) film, a heater, and a temperature sensor. The SMO film of the thin film gas sensor is an thin film (e.g., between 90 nanometers and 110 nanometers thick), and the SMO film of the bulk film gas sensor is an thick film (e.g., between 5 micrometers and 20 micrometers thick). The gas sensor device detects gases with large molecules based on a variation between resistances of the SMO thin film and the SMO thick film.

BACKGROUND Technical Field

The present disclosure is directed to a gas sensor device to detect airquality.

Description of the Related Art

Air quality is important to maintain one's health. Air pollution maylead to a variety of health issues, such as cardiopulmonary ailments.Children are particularly susceptible to air pollution. Unfortunately,air pollution is not limited to outdoor pollution. A large range ofchemical compounds can be found in indoor environments, such as inhomes, offices, and factories. For example, large concentrations ofvolatile organic compounds (VOC) and siloxanes are often detected inindoor environments.

VOCs include compounds such as ethanol, toluene, benzene, formaldehyde,tetrachloroethene (TCE), and methylene chloride. VOCs may derive from awide variety of different sources. For example, VOCs may derive from airconditioners, building materials, furniture, solvents, paint, andcarpeting. VOCs may even be caused by quotidian activities, such abreathing, cooking, and cleaning.

Siloxanes include compounds such as cyclotrisiloxane (also known as D3),cyclotetrasiloxane (also known as D4), cyclopentasiloxane (also known asD5), and cyclohexasiloxane (also known as D6), and hexamethyldisiloxane(also known as HDMS). Siloxanes are generally both toxic and persistent.For example, cyclotetrasiloxane (D4) has been categorized as a disruptorin some countries, and is a potentially reproductive toxicant that canalter human fertility. Cyclotetrasiloxane (D4) has also been shown tointerfere with human hormonal functions. As another example, exposure tohigh doses of cyclopentasiloxane (D5) has been shown to cause uterinetumors and damage the immune and reproductive systems.Cyclopentasiloxane (D5) may also affect the neurotransmitters of thenervous system. Siloxanes have also been shown to have a potential forbioaccumulation in aquatic organisms. Siloxanes may derive from varietyof everyday products, such as cosmetics and household cleaners. Forexample, hair products often include siloxanes to dry faster, anddeodorants often include siloxanes to improve application on a humanbody. Detergents and phone covers also often include siloxanes for itselastomeric properties.

Siloxane molecules are large relative to VOC molecules. Siloxanemolecules are up to eight times larger than volatile organic compounds(VOC) molecules. FIG. 1 is a chart showing the molecular weights for avariety of volatile organic compounds (VOC) and siloxanes. As shown inFIG. 1, the molecular weights of siloxanes, such as cyclotrisiloxane(D3), cyclotetrasiloxane (D4), cyclopentasiloxane (D5),cyclohexasiloxane (D6), and hexamethyldisiloxane (HDMS) are much largerthan the VOCs. For example, ethanol has a molecular weight of 46.068g/mol, while cyclopentasiloxane (D5) has a molecular weight of 370.8g/mol.

Some people are particularly sensitive to gases, such as VOCs andsiloxanes, and will experience allergic reactions, including headaches,dizziness, and irritation. However, most people are unable to detecthazardous levels of gases. Accordingly, it is important for buildings tobe equipped with gas sensors to detect harmful levels of gases.Unfortunately, due to the size of siloxane molecules, siloxanes are noteasily detected with known gas sensors. Many gas sensors are insensitiveto siloxanes, become instable when exposed to siloxanes, and/or provideinaccurate measurements.

BRIEF SUMMARY

The present disclosure is directed to a gas sensor device that detectsgases with large molecules (e.g., a molecular weight between 150 g/moland 450 g/mol), such as siloxanes. The gas sensor device may be used fora variety of applications, such as an indoor air quality sensor and anoutdoor air quality sensor.

The gas sensor device includes a thin film gas sensor and a bulk filmgas sensor. The thin film gas sensor and the bulk film gas sensor eachinclude a semiconductor metal oxide (SMO) film, a heater, and atemperature sensor. The SMO film of the thin film gas sensor is a thinfilm (e.g., between 90 nanometers and 110 nanometers thick), and the SMOfilm of the bulk film gas sensor is a thick film (e.g., between 5micrometers and 20 micrometers thick).

Due to the differences between the SMO thin film and the SMO thick film(e.g., the SMO thin film being non-porous and the SMO thick film beingporous), the SMO thin film and the SMO thick film react differently togases with large molecules. In particular, the SMO thin film willundergo very little, if any, change in resistance when exposed to a gaswith large molecules, and the SMO thick film will undergo a significantchange in resistance when exposed to a gas with large molecules. The gassensor device detects gases with large molecules, such as siloxanes,based on a variation between the resistances of the SMO thin film andthe SMO thick film.

The gas sensor device is also used to selectively detect gases that doesnot have large molecules (e.g., a gas with a molecular weight that isless than 100 g/mol), such as VOCs. The gas sensor device detects aparticular gas based on a lack of variation between the resistances ofthe SMO thin film and the SMO thick film.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar featuresor elements. The size and relative positions of features in the drawingsare not necessarily drawn to scale.

FIG. 1 is a chart showing the molecular weights for a variety ofvolatile organic compounds (VOC) and siloxanes according to anembodiment of the present disclosure.

FIG. 2 is a block diagram of a gas sensor device according to anembodiment of the present disclosure.

FIG. 3 is a cross-sectional view of a semiconductor metal oxide (SMO)thin film reacting with gas molecules according to an embodiment of thepresent disclosure.

FIGS. 4A and 4B are a cross-sectional view of an SMO thick film reactingwith gas molecules according to an embodiment of the present disclosure.

FIG. 5 is a chart showing resistances of the SMO thin film and the SMOthick film in response to being exposed to a VOC and a siloxaneaccording to an embodiment of the present disclosure.

FIG. 6 is a top view of the gas sensor device according to an embodimentof the present disclosure.

FIG. 7 is a cross-sectional view of the gas sensor device along the axisshown in FIG. 6 according to an embodiment of the present disclosure.

FIG. 8 is a cross-sectional view of the gas sensor device along the axisshown in FIG. 6 according to another embodiment of the presentdisclosure.

FIG. 9 is a flow diagram of a method of operating the gas sensor devicefor detecting a siloxane according to an embodiment of the presentdisclosure.

FIG. 10 is a flow diagram of a method of operating the gas sensor devicefor detecting a selected VOC according to an embodiment of the presentdisclosure.

FIG. 11 is an electrical circuit to measure a variation betweenresistances of the SMO thin film and the SMO thick film according to anembodiment of the present disclosure.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various aspects of thedisclosed subject matter. However, the disclosed subject matter may bepracticed without these specific details. In some instances, well-knownstructures and methods of manufacturing electronic devices have not beendescribed in detail to avoid obscuring the descriptions of other aspectsof the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thespecification are not necessarily all referring to the same aspect.Furthermore, the particular features, structures, or characteristics maybe combined in any suitable manner in one or more aspects of the presentdisclosure.

Reference throughout the specification to integrated circuits isgenerally intended to include integrated circuit components built onsemiconducting or glass substrates, whether or not the components arecoupled together into a circuit or able to be interconnected. Throughoutthe specification, the term “layer” is used in its broadest sense toinclude a thin film, a cap, or the like, and one layer may be composedof multiple sub-layers.

The present disclosure is directed to a gas sensor device that detectsgases with large molecules (e.g., a gas with a molecular weight between150 g/mol and 450 g/mol), such as siloxanes.

FIG. 2 is a block diagram of a gas sensor device 10 according to anembodiment of the present disclosure. The gas sensor device 10 detectsgases with large molecules, such as siloxanes. The gas sensor device 10includes a thin film gas sensor 12, a bulk film gas sensor 14, and aprocessor 15.

The thin film gas sensor 12 and the bulk film gas sensor 14 eachincludes a thin film that is an active sensor area, a heater, and atemperature sensor. The thin film gas sensor 12 includes such as asemiconductor metal oxide (SMO) film 16, a heater 18, and a temperaturesensor 20. The bulk film gas sensor 14 includes an SMO thick film 22, aheater 24, and a temperature sensor 26. Throughout this disclosure thefilms are referred to as SMO films, however other active sensormaterials may be used as the sensor films.

The SMO thin film 16 and the SMO thick film 22 are made of a materialthat chemically reacts with gases, such as VOCs, in a surroundingenvironment. For example, the SMO films may include tin oxide (SnO₂),zinc oxide (ZnO₂), and/or indium oxide (In₂O₃). The SMO films act asactive sensor areas. When heated to certain temperatures, the SMO filmsexperience a change in resistivity when exposed to certain gases. Forexample, when heated to between 100 degrees Celsius and 400 degreesCelsius, a resistance of a film of tin oxide changes in the presence ofmethane gas (CH₄), liquid petroleum gas (LPG), or hydrogen gas (H₂).Accordingly, a presence of a specific gas may be detected based on acurrent resistivity of the SMO films. As previously discussed, althoughthe SMO thin film 16 and the SMO thick film 22 are referred to as SMOfilms throughout this disclosure, other active sensor materials that aresensitive to gases may be used.

In one embodiment, the materials for the SMO thin film 16 and the SMOthick film 22 are selected such that the SMO thin film 16 and the SMOthick film 22 have the same resistance and sensitivity to gases, such asVOCs. For example, in one embodiment, the SMO thin film 16 and the SMOthick film 22 are made of the same material and are heated to the sametemperature. It is noted, however, that different material andtemperatures for the SMO thin film 16 and the SMO thick film 22 may beused.

The SMO thin film 16 is a thin film. The SMO thin film 16 may, forexample, be between 90 nanometers and 110 nanometers thick. Generally,SMO thin films are non-porous, and do not include any spaces orcavities. FIG. 3 is a cross-sectional view of the SMO thin film 16reacting with gas molecules 30 according to an embodiment of the presentdisclosure. As the SMO thin film 16 is non-porous, the SMO thin film 16may include some cracks, but does not include any spaces for gasmolecules 30 to pass through. As a result, the gas molecules 30 reactwith (e.g., reduce or oxidize) the SMO thin film 16 at the SMO thinfilm's 16 exposed upper surface.

The SMO thick film 22 is a thick film. The SMO thick film 22 may, forexample, be between 5 micrometers and 20 micrometers thick. Generally,in contrast to SMO thin films, SMO thick films are porous, and includespaces or cavities. FIGS. 4A and 4B are a cross-sectional view of theSMO thick film 22 reacting with gas molecules 30 according to anembodiment of the present disclosure. As the SMO thick film 22 isporous, the SMO thick film 22 includes spaces 36 for gas molecules 30 topass through or enter. As a result, the gas molecules 30 react with(e.g., reduce or oxidize) the SMO thick film 22 at the SMO thick film's22 exposed upper surface, and at surfaces within the spaces 36 of theSMO thick film 22.

Due to these differences between the SMO thin film 16 and the SMO thickfilm 22 (e.g., the SMO thin film 16 being non-porous and the SMO thickfilm 22 being porous), the SMO thin film 16 and the SMO thick film 22react differently to gases with large molecules, such as siloxanes. FIG.5 is a chart showing resistances of the SMO thin film 16 and the SMOthick film 22 in response to being exposed to a VOC and a siloxaneaccording to an embodiment of the present disclosure.

Between time t₀ and time t₁, the SMO thin film 16 and the SMO thick film22 are both unexposed to a VOC and siloxane. As a result, the SMO thinfilm 16 and the SMO thick film 22 do not react with any gas molecules,and their resistances remain unchanged. The SMO thin film 16 and the SMOthick film 22 have an initial resistance R₀.

Between time t₁ and time t₂, the SMO thin film 16 and the SMO thick film22 are both exposed to a VOC, such as ethanol, and react with the gasmolecules of the VOC. As previously discussed with respect to FIGS. 3and 4, the VOC molecules react with the SMO thin film's 16 exposed uppersurface, and the VOC molecules react with the SMO thick film's 22exposed upper surface and surfaces within the spaces 36 or cavities ofthe SMO thick film 22. As a result, the resistances of the SMO thin film16 and the SMO thick film 22 undergo a change, and drop to a resistanceR₁. It is noted that the SMO thin film 16 and the SMO thick film 22react the same to the VOC and have the same resistance R₁ because, aspreviously discussed, the SMO thin film 16 and the SMO thick film 22 aremade of the same material and are heated to the same temperature.

Between time t₂ and t₃, the VOC is released or vacated, and the SMO thinfilm 16 and the SMO thick film 22 are both once again unexposed to a VOCand siloxane. As a result, the SMO thin film 16 and the SMO thick film22 do not react with any gas molecules, and return to the initialresistance R₀.

At time t₃, the SMO thin film 16 and the SMO thick film 22 are bothexposed to a siloxane, such as cyclotrisiloxane (D3). In contrast to theexposure to the VOC between time t₁ and time t₂, the SMO thin film 16and the SMO thick film 22 react differently to the siloxane at time t₃.Namely, the resistance of the SMO thin film 16 continues to remain atthe initial resistance R₀, and the resistance of the SMO thick film 22drops to the resistance R₂.

The SMO thin film 16 and the SMO thick film 22 each react differently tothe siloxane due, at least in part, to the size of the siloxanemolecules, and to the difference in porousness between the SMO thin film16 and the SMO thick film 22. As previously discussed, the siloxanemolecules react with the exposed upper surface of the SMO thin film 16because the SMO thin film 16 is non-porous. The SMO thin film 16undergoes very little, if any, change in resistance because the siloxanemolecules are too large to react properly with the exposed uppersurface. In contrast, as previously discussed, the siloxane moleculesreact with the exposed upper surface of the SMO thin film 16 and withinthe spaces 36 within the SMO thick film 22 because the SMO thick film 22is porous. The SMO thick film 22 is able to react much more with thesiloxane molecules compared to the SMO thin film 16 because the siloxanemolecules react within the SMO thick film itself. As a result, the SMOthick film 22 will undergo a significant change in resistance.

It is noted that the resistance of the SMO thick film 22 may fluctuatewhen exposed to a siloxane. For example, as shown in FIG. 5, the SMOthick film 22 rises from the resistance R₂ to the resistance R₃, whilebeing exposed to the siloxane. The resistance of the SMO thick film 22may fluctuate for a variety of reasons. In some cases, for example, theresistance of the SMO thick film 22 fluctuates because the upper exposedsurface of the SMO thick film 22, itself, is modified by the siloxanemolecules such that the SMO thick film 22 is unable to maintain a steadyresistance. For instance, the siloxane may modify the SMO thick film 22such that the SMO thick film 22 is unable to maintain the resistance R₂.In other cases, for example, the resistance of the SMO thick film 22fluctuates because the siloxane molecules become trapped or stuck withinthe SMO thick film 22 and hinder further chemical reactions.

As the SMO thin film 16 and the SMO thick film 22 react differently tosiloxane, the SMO thin film 16 and the SMO thick film 22 can be usedcooperatively to detect siloxanes or other similar gases with largemolecules. Namely, the SMO thin film 16 and the SMO thick film 22 may beused simultaneously to gather data, and a siloxane may detected bycomparing the offset or difference in resistances between the SMO thinfilm 16 and the SMO thick film 22. The detection of gases with largemolecules, such as siloxanes, will be discussed in further detail withrespect to FIG. 9.

Returning to FIG. 2, the heaters 18, 24 heat the SMO thin film 16 andthe SMO thick film 22, respectively, to a desired temperature. In oneembodiment, the heaters 18, 24 are resistive heaters that heat the SMOthin film 16 and the SMO thick film 22 using the Joule effect, bydissipating current through a resistance. As will be discussed infurther detail with respect to FIGS. 6-7, the heaters 18, 24 underlieand heat the SMO thin film 16 and the SMO thick film 22, respectively.

As previously discussed, the SMO thin film 16 and the SMO thick film 22should be heated to a specific temperature in order to react with aspecific gas. In one embodiment, the heaters 18, 24 heat the SMO thinfilm 16 and the SMO thick film 22, respectively, to be within the sametemperature range in order to sense the same gas.

The temperature sensors 20, 26 measure the current temperature of theSMO thin film 16 and the SMO thick film 22, respectively. Thetemperature sensors 20, 26 are positioned adjacent to the SMO thin film16 and the SMO thick film 22, respectively, in order to obtain accuratemeasurements. As will be discussed in further detail below, thetemperature sensors 20, 26 are used as a feedback control device forautomatically adjusting the heaters 18, 24, respectively, to heat theSMO thin film 16 and the SMO thick film 22 to desired temperatures. Forexample, the temperature sensor 20 may measure a current temperature ofthe SMO thin film 16, and the heater 18 may adjust its temperature basedon the current temperature of the SMO thin film 16.

The processor 15 is coupled to the thin film gas sensor 12 and the bulkfilm gas sensor 14. The processor 15 is configured to activate anddeactivate each of the thin film gas sensor 12 and the bulk film gassensor 14; measure current resistivity of the SMO thin film 16 and theSMO thick film 22; control the heaters 18, 24 to heat the SMO thin film16 and the SMO thick film 22 to a particular temperature; andcommunicate with the temperature sensors 20, 26 to obtain currenttemperatures of the SMO thin film 16 and the SMO thick film 22. Theprocessor 15 may be any type of controller, microprocessor, orapplication specific integrated circuit (ASIC) that communicates withand controls the thin film gas sensor 12 and the bulk film gas sensor14. The processor may be in a same package as the gas sensors or may bea separate chip.

In one embodiment, the processor 15 reads the current temperatures ofthe SMO thin film 16 and the SMO thick film 22 via the temperaturesensors 20, 26, respectively; and then controls the heaters 18, 24 basedon the current temperature of the SMO thin film 16 and the SMO thickfilm 22, respectively. For example, the processor 15 may receive thecurrent temperature of the SMO thin film 16 via the temperature sensor20, and adjust the heater 18 to maintain a desired temperature (e.g.,between 300 degrees Celsius and 350 degrees Celsius) of the SMO thinfilm 16. By adjusting the heaters 18, 24 to specific temperatures, theprocessor 15 is able to tune the thin film gas sensor 12 and the bulkfilm gas sensor 14, more specifically the SMO thin film 16 and the SMOthick film 22, to be sensitive to a particular gas. In one embodiment,the processor 15 adjusts the heaters 18, 24 to maintain the sametemperature range such that the SMO thin film 16 and the SMO thick film22 are sensitive to the same gas.

FIG. 6 is a top view of the gas sensor device 10 according to anembodiment of the present disclosure. FIG. 7 is a cross-sectional viewof the gas sensor device 10 along the axis shown in FIG. 6 according toan embodiment of the present disclosure. It is beneficial to reviewFIGS. 6 to 7 together. It is noted that the dimensions set forth hereinare provided as examples. Other dimensions are envisioned for thisembodiment and all other embodiments of this application.

The gas sensor device 10 includes the SMO thin film 16 and the SMO thickfilm 22; the heaters 18, 24; a substrate 38; a first dielectric layer40; a second dielectric layer 42; a third dielectric layer 44; and afourth dielectric layer 46.

The first dielectric layer 40 is formed on the substrate 38 using, forexample, deposition or a growth process. The substrate 38 may be madeof, for example, silicon or glass. In one embodiment, the substrate 38has a thickness in the range of 500 micrometers to 600 micrometers. Thefirst dielectric layer 40 may be made of, for example, oxide. In oneembodiment, the first dielectric layer 40 has a thickness in the rangeof 3 micrometers to 10 micrometers.

The second dielectric layer 42 is formed on the first dielectric layer40 using, for example, deposition or a growth process. The seconddielectric layer 42 may be made of, for example, silicon nitride. In oneembodiment, the second dielectric layer 42 has a thickness in the rangeof 300 nanometers to 550 nanometers.

Cavities 48, 50 are formed between the first dielectric layer 40 and thesecond dielectric layer 42. As best shown in FIG. 7, the thin film gassensor 12 and the bulk film gas sensor 14 each includes a respectivecavity. Namely, the thin film gas sensor 12 includes the cavity 48, andthe bulk film gas sensor 14 includes the cavity 50. The cavities 48, 50may be formed by patterning recesses 52 in the first dielectric layer 40using, for example, photolithography and etching techniques; and fillingthe recesses 52 with a sacrificial material 54, such as polyimide. Thesecond dielectric layer 42 may then be formed on the first dielectriclayer 40 and the sacrificial material 54. Portions of the sacrificialmaterial 54 may then be removed using, for example, photolithography andetching techniques. Remaining portions of the sacrificial material 54,as best shown in FIG. 7, provide additional support for portions of thesecond dielectric layer 42 overlying the cavities 48, 50. In oneembodiment, each of the cavities 48, 50 has a depth in the range of 2micrometers to 5 micrometers.

The cavities 48, 50 provide air gaps between the first dielectric layer40 and the second dielectric layer 42. As air has low thermalconductivity, the cavities 48, 50 provide thermal insulation and confineheat within the thin film gas sensor 12 and the bulk film gas sensor 14.As a result, temperatures of the SMO thin film 16 and the SMO thick film22 may be maintained with less power. In addition, as polyimide also haslow thermal conductivity, using polyimide for the sacrificial material54 provides additional thermal insulation for the thin film gas sensor12 and the bulk film gas sensor 14.

The heaters 18, 24 are formed on the second dielectric layer 42 using,for example, deposition. The heaters 18, 24 directly overlie thecavities 48, 50, respectively. As previously discussed, in oneembodiment, the heaters 18, 24 are resistive heaters that heat the SMOthin film 16 and the SMO thick film 22 using the Joule effect bydissipating current through a resistance. In this embodiment, theheaters 18, 24 include a resistive layer 56, such as tantalum aluminum.In one embodiment, the resistive layer 56 has a thickness in the rangeof 100 nanometers to 200 nanometers. Although a single resistive layeris shown in FIG. 7, the heaters 18, 24 may include a plurality ofresistive layers. In one embodiment, the heaters 18, 24 include at leastone resistive layer on both sides of the SMO thin film 16 and the SMOthick film 22 such that at least a portion of the SMO thin film 16 andthe SMO thick film 22 are sandwiched between two resistive layers.

The third dielectric layer 44 is formed on the second dielectric layer42 and the heaters 18, 24 using, for example, deposition or a growthprocess. The third dielectric layer 44 may be made of, for example,silicon nitride. In one embodiment, the third dielectric layer 44 has athickness in the range of 200 nanometers to 400 nanometers.

The SMO thin film 16 and the SMO thick film 22 are formed on the thirddielectric layer 44 using, for example, deposition. The SMO thin film 16and the SMO thick film 22 may be formed by forming an SMO layer on thethird dielectric layer 44 and patterning the SMO layer using, forexample, photolithography and etching techniques. As previouslydiscussed, SMO thin film 16 and the SMO thick film 22 are made of amaterials that chemically react with various gases in a surroundingenvironment. For example, the SMO thin film 16 and the SMO thick film 22may include tin oxide (SnO₂), zinc oxide (ZnO₂), and/or indium oxide(In₂O₃). In one embodiment, the SMO thin film 16 has a thickness in therange of 50 nanometers to 150 nanometers. In one embodiment, the SMOthick film 22 has thickness in the range of 300 nanometers to 550nanometers.

A fourth dielectric layer 46 is formed on the third dielectric layer 44and the SMO thin film 16 and the SMO thick film 22 using, for example,deposition or a growth process. The fourth dielectric layer 46 ispatterned using, for example, photolithography and etching techniques toexpose the SMO thin film 16 and the SMO thick film 22 such that the SMOthin film 16 and the SMO thick film 22 are exposed to a surroundingenvironment, as shown in FIG. 7. The fourth dielectric layer 46 may bemade of, for example, silicon nitride. In one embodiment, the fourthdielectric layer 46 has a thickness in the range of 300 nanometers to550 nanometers.

In the embodiment shown in FIGS. 6 to 7, the thin film gas sensor 12 andthe bulk film gas sensor 14 are both formed on the same die. Namely, thethin film gas sensor 12 and the bulk film gas sensor 14 are formed onthe substrate 38. In another embodiment, the thin film gas sensor 12 andthe bulk film gas sensor 14 are formed on separate dies.

It is noted that the processor 15 and the temperature sensors 20, 26 arenot shown in FIGS. 6 to 7 for simplicity purposes. In one embodiment,the thin film gas sensor 12 and the bulk film gas sensor 14 and theprocessor 15 are all formed on the same substrate. In one embodiment,the processor 15 is formed on a separate substrate from the thin filmgas sensor 12 and the bulk film gas sensor 14, and is electricallycoupled to the thin film gas sensor 12 and the bulk film gas sensor 14via an interconnect. In one embodiment, the temperature sensors 20, 26are positioned adjacent to the SMO thin film 16 and the SMO thick film22, respectively, in order to obtain accurate measurements. In addition,although not shown in FIGS. 6 to 7, the gas sensor device 10 may includea plurality of conductive layers that electrically couple the SMO thinfilm 16, the SMO thick film 22, and the heaters 18, 24 to the processor15 and/or other electrical components (e.g., transistors, capacitors,resistors, etc.).

FIG. 8 is a cross-sectional view of the gas sensor device along the axisshown in FIG. 6 according to another embodiment of the presentdisclosure.

In the embodiment shown in FIG. 7, the thin film gas sensor 12 and thebulk film gas sensor 14 each includes a respective cavity. That is, thethin film gas sensor 12 includes the cavity 48, and the bulk film gassensor 14 includes the cavity 50. In contrast, in the embodiment shownin FIG. 8, the thin film gas sensor 12 and the bulk film gas sensor 14share a single cavity 58. Similar to the cavities 48, 50, the cavity 58provides an air gap between the first dielectric layer 40 and the seconddielectric layer 42 to provide thermal insulation for the thin film gassensor 12 and the bulk film gas sensor 14. As a result, temperatures ofthe SMO thin film 16 and the SMO thick film 22 may be maintained withless power. By using the cavity 58 instead of the multiple cavities 48,50, fabrication of the gas sensor device 10 is also simplified. Inaddition, the cavity 58 provides a larger air gap for increased thermalinsulation.

As previously discussed, the SMO thin film 16 and the SMO thick film 22react differently to siloxanes. The SMO thin film 16 will undergo verylittle, if any, change in resistance when exposed to a siloxane, and theSMO thick film 22 will undergo a significant change in resistance whenexposed to a siloxane. Thus, siloxanes may be detected based on avariation between the resistances of the SMO thin film 16 and the SMOthick film 22.

FIG. 9 is a flow diagram of a method 60 of operating the gas sensordevice 10 for detecting a siloxane according to an embodiment of thepresent disclosure.

In block 62, the thin film gas sensor 12 is ON and the bulk film gassensor 14 is ON. In particular, the processor 15 activates the thin filmgas sensor 12 and the bulk film gas sensor 14 simultaneously. When thethin film gas sensor 12 and the bulk film gas sensor 14 are activated,the heaters 18, 24 are turned on to heat the SMO thin film 16 and theSMO thick film 22, respectively, to a desired temperature. In oneembodiment, as previously discussed, the SMO thin film 16 and the SMOthick film 22 are heated to the same temperature. In addition, theprocessor 15 monitors the resistances of the SMO thin film 16 and theSMO thick film 22.

In decision branch 64, the processor 15 determines whether a resistancevariation (Z) between the SMO thin film 16 and the SMO thick film 22 isgreater than or equal to a predetermine threshold. For example, indecision branch 64, the processor 15 may determine whether theresistance variation (Z) is greater than 5%. It is noted that anypredetermined threshold may be used.

In one embodiment, the resistance variation (Z) is determined using areference ratio (CST) of the SMO thin film 16 and the SMO thick film 22,and a current ratio (VAR) of the SMO thin film 16 and the SMO thick film22. The resistance variation (Z) may be calculated using equation (1):

$\begin{matrix}{Z = \frac{{CST} - {VAR}}{CST}} & (1)\end{matrix}$

The reference ratio (CST) is a defined constant that is used as areference value. The reference ratio (CST) is determined beforehand byexposing the SMO thin film 16 and the SMO thick film 22 to a selectedgas, such as a VOC, having a known concentration range. The referenceratio (CST) is a ratio between (1) a change of resistance(ΔRthin_(reference)) of the SMO thin film 16 in response to a firstchange in gas concentration (ΔC₁) of the selected gas, divided by thefirst change in gas concentration (ΔC₁); and (2) a change of resistanceof the SMO thick film 22 (ΔRthick_(reference)) in response to a secondchange in gas concentration (ΔC₂) of the selected gas, divided by thesecond change in gas concentration (ΔC₂). The reference ratio (CST) maybe calculated using equation (2):

$\begin{matrix}{{CST} = \frac{\Delta\;{Rthin}_{reference}\text{/}\Delta\; C_{1}}{\Delta\;{Rthick}_{reference}\text{/}\Delta\; C_{2}}} & (2)\end{matrix}$In one embodiment, the first gas concentration range (ΔC₁) is equal tothe second gas concentration range (ΔC₂).

The current ratio (VAR) is calculated similar to the reference ratio(CST), except that the current ratio (VAR) is calculated using currentlymeasured resistances of the SMO thin film 16 and the SMO thick film 22.In particular, the current ratio (VAR) is a ratio between (1) a currentchange of resistance (ΔRthin_(current)) of the SMO thin film 16 dividedby the first gas concentration range (ΔC₁), and (2) a current change ofresistance of the SMO thick film 22 (ΔRthick_(current)) divided by thesecond gas concentration range (ΔC₂). The current ratio (VAR) may becalculated using equation (3):

$\begin{matrix}{{VAR} = \frac{\Delta\;{Rthin}_{current}\text{/}\Delta\; C_{1}}{\Delta\;{Rthick}_{current}\text{/}\Delta\; C_{2}}} & (3)\end{matrix}$As previously discussed, the current ratio (VAR) is compared to thereference ratio (CST) using equation (1) to determine the resistancevariation (Z) between the SMO thin film 16 and the SMO thick film 22.

If the variation (Z) is less than the predetermined threshold, theprocessor 15 determines that a siloxane is not present, and the method60 returns to step 62 to continue monitoring for siloxanes. If thevariation (Z) is greater than or equal to the predetermined threshold,the method 60 moves to block 66.

In block 66, the processor 15 determines that siloxane is present. Inone embodiment, when the processor 15 determines that siloxane ispresent, the gas sensor device 10 sounds an alarm to alert a user. Themethod 60 then moves to decision branch 68.

In decision branch 68, the processor 15 determines whether theresistance variation (Z) between the SMO thin film 16 and the SMO thickfilm 22 is less than the predetermine threshold. For example, indecision branch 68, the processor 15 may determine whether theresistance variation (Z) is less than 5%.

If the variation Z is greater than or equal to the predeterminedthreshold, siloxane is still present and the method 60 returns to block66. If the variation Z is less than the predetermined threshold, theprocessor 15 determines that a siloxane is no longer present, and themethod 60 moves to block 70.

In block 70, a renewing process is performed. As previously discussed,the SMO thick film 22 is porous and includes the spaces 36. Becausesiloxane molecules are much larger than VOC molecules, the siloxanemolecules often become stuck within the spaces 36 of the SMO thick film22 and are unable to release from the SMO thick film 22. The renewingprocess releases any siloxane molecules that are stuck within the SMOthick film 22. In one embodiment, the renewing process is a burningsequence that heats the SMO thick film 22 to a high temperature (e.g.,between 600 degrees Celsius and 800 degrees Celsius) to input energy into the SMO thick film 22 and cause siloxane molecules to release fromthe SMO thick film 22.

Once the renewing process is complete, the method 60 returns to block62, where the gas sensor device 10 begins to monitor for siloxanesagain. In one embodiment, the renewing process is also performed on theSMO thin film 16.

It is noted that, although the flow diagram of the method 60 shown inFIG. 9 is discussed with respect to detecting siloxanes, the method 60may be used to detect other gases with large molecules (e.g., a gas witha molecular weight between 150 g/mol and 450 g/mol).

As discussed with respect to FIG. 9, the gas sensor device 10 may beoperated to detect siloxanes based on a variation between theresistances of the SMO thin film 16 and the SMO thick film 22. However,the gas sensor device 10 may also be operated to detect a specific gasthat does not have large molecules (e.g., a gas with a molecular weightthat is less than 100 g/mol), such as a VOC. For example, the gas sensordevice 10 may be operated to specifically detect ethanol. As previouslydiscussed, the SMO thin film 16 and the SMO thick film 22 react the sameto gases, such as VOCs, when the SMO thin film 16 and the SMO thick film22 are made of the same material and are heated to the same temperature.Thus, a selected gas may be detected based on a lack of variationbetween the resistances of the SMO thin film 16 and the SMO thick film22 (i.e., the resistances of the SMO thin film 16 and the SMO thick film22 reacting the same).

FIG. 10 is a flow diagram of a method 80 of operating the gas sensordevice 10 for detecting a selected VOC, such as ethanol, according to anembodiment of the present disclosure.

In block 82, similar to block 62 of the method 60, the thin film gassensor 12 is ON and the bulk film gas sensor 14 is ON. In particular,the processor 15 activates the thin film gas sensor 12 and the bulk filmgas sensor 14 simultaneously. When the thin film gas sensor 12 and thebulk film gas sensor 14 are activated, the heaters 18, 24 are turned onto heat the SMO thin film 16 and the SMO thick film 22, respectively, toa desired temperature. In one embodiment, as previously discussed, theSMO thin film 16 and the SMO thick film 22 are heated to the sametemperature. In addition, the processor 15 monitors the resistances ofthe SMO thin film 16 and the SMO thick film 22.

In decision branch 84, the processor 15 determines whether the currentratio (VAR) of the SMO thin film 16 and the SMO thick film 22 issubstantially equal to the reference ratio (CST) of the SMO thin film 16and the SMO thick film 22.

As previously discussed, the reference ratio (CST) is a defined constantthat is used as a reference value, and is determined beforehand using aselected gas having a known concentration range. The reference ratio(CST) is calculated using equation (2) above. For the embodiment of FIG.10, the reference ratio (CST) is determined using the selected VOC.

As previously discussed, the current ratio (VAR) is calculated similarto the reference ratio (CST), except that the current ratio (VAR) iscalculated using currently measured resistances of the SMO thin film 16and the SMO thick film 22. The current ratio (VAR) is calculated usingequation (3) above.

If the current ratio (VAR) is not substantially equal (e.g., greaterthan a 1% difference) to the reference ratio (CST), the processor 15determines that the selected VOC is not present, and the method 80returns to step 82 to continue monitoring for the selected VOC. If thecurrent ratio (VAR) is substantially equal (e.g., less than a 1%difference) to the reference ratio (CST), the method 80 moves to block86.

In block 86, the processor 15 determines that the selected VOC ispresent. In one embodiment, when the processor 15 determines thatselected VOC is present, the gas sensor device 10 sounds an alarm toalert a user. The method 80 then moves to decision branch 88.

In decision branch 88, the processor 15 once again determines whetherthe current ratio (VAR) of the SMO thin film 16 and the SMO thick film22 is substantially equal to the reference ratio (CST) of the SMO thinfilm 16 and the SMO thick film 22.

If the current ratio (VAR) is substantially equal (e.g., greater than a1% difference) to the reference ratio (CST), the selected VOC is stillpresent, and the method 80 returns to block 86. If the current ratio(VAR) is substantially equal (e.g., less than a 1% difference) to thereference ratio (CST), the processor 15 determines that the selected VOCis no longer present, and the method 80 moves to block 90.

In block 90, similar to block 70, a renewing process is performed. Aspreviously discussed, the renewing process releases any molecules thatare stuck within the SMO thick film 22. Once the renewing process iscomplete, the method 80 returns to block 82, where the gas sensor device10 begins to monitor for the selected VOC again. In one embodiment, therenewing process is also performed on the SMO thin film 16.

It is noted that, although the flow diagram of the method 80 shown inFIG. 10 is discussed with respect to detecting a selected VOC, themethod 80 may be used to detect other gases that do not have largemolecules (e.g., a gas with a molecular weight that is less than 100g/mol).

FIG. 11 is an electrical circuit 92 to measure a variation betweenresistances of the SMO thin film 16 and the SMO thick film 22 accordingto an embodiment of the present disclosure.

The electrical circuit 92 includes the SMO thin film 16, represented bya resistor Rthin; the SMO thick film 22, represented by a resistorRthick; and resistors R1, R2. The resistances of the resistor R1, R2 areknown. The resistor Rthin, the resistor Rthick, and the resistors R1, R2are arranged as a Wheatstone half bridge. In particular, the resistorRthick is electrically coupled between nodes A and D, the resistor Rthinis electrically coupled between nodes A and B, the resistor R1 iselectrically coupled between nodes D and C, and the resistor R2 iselectrically coupled between nodes B and C.

As previously discussed, the SMO thin film 16 and the SMO thick film 22reacts differently to siloxanes. The SMO thin film 16 will undergo verylittle, if any, change in resistance when exposed to a siloxane, and theSMO thick film 22 will undergo a significant change in resistance whenexposed to a siloxane. As the resistance of the SMO thin film 16 willnot change, the resistor Rthin may be used as a witness resistance inthe Wheatstone bridge of the electrical circuit 92. That is, when aninput voltage Vin is applied between nodes A and C, the output voltageVout will be proportional to a change in resistance of the SMO thickfilm 22 (i.e., the resistor Rthick.

The various embodiments provide a gas sensor device that detects gaseswith large molecules, such as siloxanes. The gas sensor device may alsobe used to selectively detect other gases, such as VOCs.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A device, comprising: a substrate; a firstgas sensor on the substrate, the first gas sensor including a firstsemiconductor metal oxide (SMO) film, the first SMO film having a firstthickness; a second gas sensor on the substrate, the second gas sensorincluding a second SMO film, the second SMO film having a secondthickness that is greater than the first thickness, the second SMO filmbeing electrically coupled to the first SMO film; a first resistorelectrically coupled to the second SMO film; and a second resistorelectrically coupled to the first resistor and the first SMO film, thefirst and second SMO films and the first and second resistors forming aWheatstone half bridge circuit.
 2. The device of claim 1 wherein thefirst gas sensor includes a first heater configured to heat the firstSMO film, and the second gas sensor includes a second heater configuredto heat the second SMO film.
 3. The device of claim 1 wherein the firstgas sensor includes a first temperature sensor, and the second gassensor includes a second temperature sensor.
 4. The device of claim 1wherein the first SMO film and the second SMO film are made of the samematerial.
 5. The device of claim 1 wherein the first SMO film isnon-porous, and the second SMO film is porous.
 6. The device of claim 1wherein the first SMO film has a first resistance that changes inresponse to being exposed to a gas, and the second SMO film has a secondresistance that changes in response to being exposed to the gas.
 7. Thedevice of claim 1 wherein the first SMO film has a first resistance thatremains the same in response to being exposed to a gas, and the secondSMO film has a second resistance that changes in response to beingexposed to the gas.
 8. The device of claim 1 wherein the first thicknessis between 90 nanometers and 110 nanometers, and the second thickness isbetween 5 micrometers and 20 micrometers.
 9. A device, comprising: asubstrate; a first dielectric layer on the substrate; a seconddielectric layer on the first dielectric layer; a first cavity formed bythe first dielectric layer and the second dielectric layer; a secondcavity formed by the first dielectric layer and the second dielectriclayer; a first heater on the second dielectric layer and directlyoverlying the first cavity; a second heater on the second dielectriclayer and directly overlying the second cavity; a third dielectric layeron the first heater and the second heater, the first heater and thesecond heater being separated from each other by a portion of the thirddielectric layer; a first semiconductor metal oxide (SMO) film on thethird dielectric layer and directly overlying first heater, the firstSMO film having a first thickness; a second SMO film on the thirddielectric layer and directly overlying second heater, the second SMOfilm having a second thickness that is greater than the first thickness;and a fourth dielectric layer on the third dielectric layer, fourthdielectric layer including first and second openings that expose thefirst and second SMO films, respectively, to a surrounding environment,the first SMO film and the second SMO film being separated from eachother by a portion of the fourth dielectric layer.
 10. The device ofclaim 9, further comprising a processor configured to control the firstheater and the second heater.
 11. The device of claim 9, furthercomprising: a first temperature sensor on the substrate adjacent to thefirst SMO film; and a second temperature sensor on the substrateadjacent to second SMO film.
 12. The device of claim 9 wherein the firstSMO film and the second SMO film are made of the same material.
 13. Thedevice of claim 9 wherein the first SMO film is non-porous, and thesecond SMO film is porous.
 14. A device, comprising: a substrate; afirst dielectric layer on the substrate; a second dielectric layer onthe first dielectric layer; a cavity formed by the first dielectriclayer and the second dielectric layer; a first heater on the seconddielectric layer; a second heater on the second dielectric layer; athird dielectric layer on the first heater and the second heater; afirst film on the third dielectric layer, the first film configured toreact to a gas, the first heater configured to heat the first film; afirst temperature sensor configured to measure a temperature of thefirst film; a second film on the third dielectric layer, the second filmconfigured to react to the gas, the first film and the second filmhaving different thicknesses, the second heater configured to heat thesecond film; and a second temperature sensor configured to measure atemperature of the second film, wherein the first film, the firstheater, the second film, and the second heater directly overlies thecavity.
 15. The device of claim 14 wherein the first film and the secondfilm are made of the same material.
 16. The device of claim 14 whereinthe first film is non-porous, and the second film is porous.
 17. Thedevice of claim 14, further comprising: a fourth dielectric layer on thethird dielectric layer, the fourth dielectric layer including: a firstopening that exposes the first film to a surrounding environment; and asecond opening that exposes the second film to the surroundingenvironment.
 18. The device of claim 9 wherein the portion of the fourthdielectric layer physically contacts the first SMO film and the secondSMO film.
 19. The device of claim 9, further comprising: a firstresistor electrically coupled to the second SMO film; and a secondresistor electrically coupled to the first resistor and the first SMOfilm, the first and second SMO films and the first and second resistorsforming a Wheatstone half bridge circuit.
 20. The device of claim 14,further comprising: a first resistor electrically coupled to the secondfilm; and a second resistor electrically coupled to the first resistorand the first film, the first and second films and the first and secondresistors forming a Wheatstone half bridge circuit.